The goal of the proposed research is to develop a fundamental and predictive understanding of processes in which electron transfer and proton transfer are coupled. Such proton-coupled electron transfer (PCET) processes are critical in many areas of biology, from bioenergetics to the catalytic cycles of numerous metalloenzymes, to the trapping of reactive oxygen species. The studies proposed here focus on reactions in which one proton and one electron transfer in a single kinetic step, termed concerted proton and electron transfer (CPET). One set of projects are designed to build a unified picture of hydrogen atom transfer (HAT) reactions. These are processes which involve the concerted transfer of an electron and a proton from a single donor to a single acceptor. Model systems are being developed to develop the general principles of HAT and to understand specifically the reactivity of iron-histidine cofactors, including quinone oxidation by the Rieske iron-sulfur cluster in the mitochondrial bc1 complex and the chemistry of bis(histidine)-ligated hemes. A second set of projects examine `separated CPET' processes, in which the electron and proton transfer to different acceptors. Such reactions have received little attention but are likely to be involved in a wide range of biological redox reactions. The best-studied example is the formation of the tyrosyl-Z radical in photosystem II; models for this process are proposed. In each of these areas, some systems model specific biochemical reactions while others are designed to probe general principles. For example, the proposed studies of oxidation of phenol-base compounds will probe how framework motions affect hydrogen transfers, an issue of much current debate regarding the origins of enzymatic catalysis. These studies will provide connections to current theoretical treatments of CPET, building on our demonstration that a range of hydrogen atom transfer reactions follow Marcus theory. Brought together, these studies will develop the principles of coupled proton-electron transfers and produce new insights into a wide range of biological processes. Such fundamental principles of chemical processes that occur in biology are part of the foundation for biomedical advances. For instance, the detailed knowledge available for electron transfer reactions has proven to be of great importance in biology. The work proposed aims to build a similarly valuable understanding for reactions that involve coupled transfers of electrons and protons. PUBLIC HEALTH RELEVANCE: This project is developing a predictive understanding of a class of chemical processes that occur widely in biology but have received little study: coupled transfers of electrons and protons. These chemical reactions are central to fields as diverse as bioenergetics and the action of antioxidants. A fundamental understanding of these chemical processes is a part of the foundation on which biomedical advances will be built. [unreadable] [unreadable] [unreadable]